Functional Calibration

Richard Baker

Professor of Clinical Gait Analysis

Blog: wwRichard.net

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Model Calibration

Scaling the model to fit the person we are

measuring

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Kinematic fitting

(Inverse kinematics, Global optimisation)

Fitting a model to the marker trajectories

captured during the movement being

analysed.

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Kinematic fitting4

Calibration

• Where are the joint

centres?

• Where are the joint

axes?

• Where are the

markers?

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Static calibration – hip

Assume

• hip joint centres can be estimated from position of pelvic markers.

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Static calibration - knee7

Assume

• knee joint centres is half-way between condyles

• line between condyles defines femur coronal plane

Static calibration - ankle8

Assume

• anklle joint centres is half-way between malleoli

• line between malleioli defines tibia coronal plane

Static calibration - foot9

Assume

• long axis is parallel ot the floor

• long axis is parallel to line from heel to tip of second ray

Problems - Hip1

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Bell AL, et al. (1990). Journal of Biomechanics 23:617-621Davis R et al. (1991). Hum. Movemt Sci. 10:575-587.Leardini A et al. (1999). Journal of Biomechanics 32:99-103

Problems - Knee1

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Functional calibration

Use data from movement trials to calibrate

the model

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Hip calibration1

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Hip calibration1

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Knee calibration1

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Knee calibration1

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Functional calibration

• Growing number of different mathematical

techniques to determine joint centres and

axes.

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Does it work?

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Errors in mm (with standard deviations)

Compared to RSA Functional Bell Davis

Anterior-posterior 3.8 (6.1) -2.5 (5.9) -11.6 (16.7)

Medio-lateral -2.1 (3.7) 5.3 (9.7) 16.5 (10.2)

Proximal-distal 2.5 (5.9) -18.7 (8.2) 8.2 (9.7)

Total 11.8 (4.1) 23.2 (10.3) 29.1 (7.6)

Wrong regression equations?2

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3d- ultrasound2

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Sangeux, M., A. Peters, and R. Baker, Hip joint centre localization: Evaluation on normal subjects in the

context of gait analysis. Gait Posture, 2011. 34(3): p. 324-8.

PiG HAR Geometrical Algebraic CTT SCoRE Kylie

Sphere Transformational

REGRESSION LOCAL GLOBAL

Hip validation – 19 adults

PiG HAR Geometrical Algebraic CTT SCoRE Kylie

Sphere Transformational

REGRESSION LOCAL GLOBAL

Hip validation – 19 adults

In healthy adults functional calibration performs about as well as Harrison’s regression equations

Hip validation – 46 Children with CP

REGRESSION

In children with CP Harrington’s regression equations outperform functional calibration

Knee calibration

• Much bigger problem?

• Much more difficult to find gold standard

• Informal data suggests that skilled marker

placers can be reasonably accurate

(particularly using medial knee markers).

• Functional calibration is at least as

repeatable as they are.

• Should be independent of experience?

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Summary

• Functional calibration of hip does not

appear to offer much over the correct

regression equations.

• Functional calibration of knee should be

more repeatable for most marker placers.

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Practicalities

• None of the manufacturers have

implemented easy to use software for

functional calibration.

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Thanks for listening

Richard Baker

Professor of Clinical Gait Analysis

Blog: wwRichard.net

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